Processes for obtaining direct positive images which comprise carrying out a surface treatment of an inner latent image type silver halide photographic emulsion in the presence of a nucleus forming agent and photographic emulsions and light-sensitive materials used for such processes are disclosed in U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,675,318 and 3,227,552, British Pat. No. 1,151,363, Japanese Patent Publication No. 29405/68, U.S. Pat. No. 2,592,250 and British Pat. No. 1,011,062, etc.
The term "inner latent image type silver halide photographic emulsion" means a silver halide photographic emulsion having sensitive nuclei which are mainly present in the inner part of the silver halide particles so that latent images are mainly formed in the inner part of the silver halide particles based on such inner sensitive nuclei. Using such inner latent image type silver halide photographic emulsions, positive images can be directly obtained by carrying out surface development in the presence of a nucleus forming agent which is contained in the sensitive material or in a processing solution after imagewise exposed to light.
In silver halide photographic light-sensitive materials, silver halide photograhic emulsions are generally spectrally sensitized. Particularly in color light-sensitive materials, spectral sensitization is necessary to obtain a green-sensitive layer and a red-sensitive layer together with a layer which is sensitive to blue light. As useful sensitizing dyes, there are those described in, for example, German Pat. No. 929,080, U.S. Pat. Nos. 2,493,748, 2,503,776, 2,519,001, 2,912,329, 3,656,959, 3,672,897, 3,694,217, 4,025,349 and 4,046,572, British Pat. No. 1,242,588 and Japanese Patent Publication Nos. 14030/69 and 24844/77.
These sensitizing dyes may be used alone or combinations thereof may be used. Combinations of sensitizing dyes are frequently used for supersensitization. Typical examples thereof are described in U.S. Pat. Nos. 2,668,545, 2,977,229, 3,397,060, 3,522,052, 3,527,641, 3,617,293, 3,628,964, 3,666,480, 3,672,898, 3,679,428, 3,703,377, 3,814,609, 3,837,862 and 4,026,707, British Pat. Nos. 1,344,281 and 1,507,803, Japanese Patent Publication Nos. 4936/68 and 12375/78 and Japanese Patent Application (OPI) Nos. 110618/77 and 109925/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
With combinations of sensitizing dyes as described above, since dyes having different absorption wavelength are generally combined, the resultant color sensitization region is expanded in many cases as compared with the case of using one sensitizing dye. Further, if a sensitizing dye is used alone in a large amount, sufficient sensitivity cannot be obtained and frequently there is caused an increase of the minimum image density or a decrease of the maximum image density.
Further, as described in P. B. Gilman, Jr., Photogr. Sci. Eng., 20(3), 97 (1976) and U.S. Pat. No. 3,622,316, it has been attempted to obtain higher sensitivity by adsorbing sensitizing dyes onto silver.halide particles so as to form layers. According to such methods, "dye positive holes" formed in an inner dye layer as transferred to the dye in an outer dye layer and/or light energy (or photoelectrons) formed on the outer dye layer is transferred to the silver halide through the inner dye layer. In these cases, there is a restriction on the energy level of dyes adsorbed in the form of layers and, in the case that the lower or inner layer is a cationic dye and the upper or outer layer is an anionic dye as proposed by P. B. Gilman, Jr., sensitivity does not increase, rather is deteriorated, when a surface treatment is carried out in the presence of a nucleus forming agent to obtain direct positive images. Further, in the case of U.S. Pat. No. 3,622,316, there are the disadvantages that not only is the increase of sensitivity insufficient but also the sensitization wavelength region is unnecessarily expanded because of using a combination of dyes having different wavelengths, and, consequently, sharp color images cannot be obtained.